The present invention relates to a radio transceiver for providing duplex radio communication by alternately transmitting and receiving on a time division basis by use of the same frequency.
Such a method of alternate transmission and reception in a time division fashion is called a time-division duplex (TDD) system or a pingpong system. This system has advantages that restrictions imposed on frequency allotment are lessened since the same frequency is used for both transmission and reception and that high control efficiency is obtained since all interferences between transmission and reception can be monitored even in the case where dynamic channel selection is made.
In a two-frequency duplex system, which is usually used for duplex radio communication, the transmission frequency and the reception frequency of one of opposing radio stations has a reverse relation with respect to the transmission frequency and the reception frequency of the other radio station. Therefore, two kinds of equipments are required and a parent and child relation results between the two kinds of equipments. On the other hand, the TDD system is an excellent system in which the same construction can be used in both of opposing radio equipments and communication between child equipments is also possible. Further, since the transmission frequency and the reception frequency are the same, the TDD system has an advantage that even when one equipment is not provided with a diversity circuit, a pseudo diversity effect can be expected if the other equipment is provided with two antennae and makes transmission by use of one of the two antennae which exhibits a better characteristic in a reception state.
Furthermore, the TDD system is applicable not only to duplex operation by the time division multiplexing of a single radio-wave channel, but also to time-division multiple-access (TDMA) communication in which a plurality of communications are made on the time division basis. The optimization of the TDD system for use for digital codeless telephone sets in offices having a large amount of traffic can be realized as a TDMA/TDD system by arranging the transmission and reception timings alternately with a modulation rate twice as high as that in the two-frequency duplex system.
However, when a transceiver is actually fabricated in accordance with the TDD system, expected performance often cannot be sufficiently attained due to various limitations inherent in the TDD system. One of the essential problems lies in the fact that it is very difficult to completely cut a transmission wave when transmission is to be stopped in the reception state. This is because if an oscillator, such as a quartz crystal or a synthesizer is once stopped, it takes time before it will stably oscillate again and hence it is hard to catch up with the timing of time-division transmission/reception in a high-speed pingpong system. In the conventional TDD system, therefore, the oscillation at the transmission frequency or a frequency in the vicinity is maintained in the transceiver in the reception state, and deterioration of reception sensitivity due to the transmission wave is prevented by cutting off a power source for a part of circuit or shielding the same. However, the countermeasure such as shielding has limitations in its performance. Also, if the frequency of the oscillator is shifted to an adjacent channel in the reception state to prevent the deterioration of reception sensitivity, there is a problem of causing interference in other transceivers or equipments of the same kind making communications in the neighborhood.